Here's my quick guide to getting all the higher level books in the game.
This does not include how to get these with tokens from the Anniversary Pack.

Lvl 79 skills:
These each require 20 Apocalypse Pages and 10 medium ink. Apocalypse pages drop in TT2-2, TT3-1, and FB89 from the mobs. The books can be made in heaven's tear of the past and in 1k streams.

Each of the 89-99 skills come in demon and sage variants.

Lvl 89 skills (fb89):
Of the lvl 89 skills available to you, 3 will drop in either Brimstone Pit (sage fb89) or Eden (demon fb89). These are the only lvl 89 skills that are relatively easy to get. On the average, each boss will drop a book 50% of the time.

Lvl 89 skills (fb99):
All other lvl 89 books drop from the last boss in either fb99 (either Hellfire Abomination or Peachblossom Ritualist). There is a 10% chance of either boss dropping 1 book. There are 96 different lvl 89 books that they can drop (48 sage and 48 demon). With 6 classes and 2 orientations thats about a 1% chance he will drop something you can use.

Lvl 92 skills:
The way to get these is to craft the books from 20 Old Book Pages.
These come from the bosses in God's Rebirth Delta (level 86+ requirement). The bosses start dropping these at stage 4 and drop on average 1 page each I think.

An Old Book Page can also be obtained by forging 10 Book Credit Cards in the cube. Book Credit Cards are found as rewards from several of the 51-60 rooms, starting at lv.96.

Lvl 99 skills:
Lvl 99 skill books appear to only come from running the cube of fate. At lvl 90+ you receive a Page of Fate as a reward for winning the cube. With 30 pages of fate you can buy a book that gives you a random book among your lvl99 books.

A page of fate can also be made out of 10 Book Credit Cards.
Lvl 100 skills:
These skills do not come in demon / sage variants. For these you need 20 Chrono Pages and 10 medium ink to craft. Chrono Pages drop from the lvl 150 world bosses that appear around the world. They also have a small chance to drop from card bosses, mini world bosses, and mobs in the past maps.

There may be some exceptional ways to get books via the Tournament or the "Hall of Blasphemy" but I am not sure if those are currently attainable.